Ceramic superconductor having a heterogeneous composition distribution and method of preparing the same

ABSTRACT

A Bi-Sr-Ca-Cu-O ceramic superconductor contains 0112 phases which are finely dispersed in a 2212-phase matrix with its c-axis oriented perpendicular to a growth direction. A method of preparing a Bi-Sr-Ca-Cu-O ceramic superconductor comprises the steps of growing crystals under conditions satisfying: G/R&gt;/=1 and GxR&gt;/=10000 where G (K/cm) represents the temperature gradient at a solid-liquid interface and R (mm/h) represents the rate of crystal growth, and annealing the grown crystals in an atmosphere having oxygen partial pressure of at least 0.05 atm. within a temperature-range of 800 DEG  to 860 DEG  C. for at least 2 hours.

This is a continuation of application Ser. No. 08/033,989, filed Mar.19, 1993, now U.S. Pat. No. 5,403,818, which application is acontinuation of Ser. No. 07/701,397, filed May 16, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconductor of a ceramicsuperconductive material and a method of preparing the same.

2. Description of the Background Art

Ceramic superconductive materials have been recently watched withinterest as materials which exhibit high critical temperatures. In orderto obtain a superconductor having a desired configuration from such aceramic superconductive material, a sintering method of press-molding araw material of ceramic powder and thereafter sintering the same isgenerally employed.

However, in such a sintering method, it is difficult to obtain a densesuperconductor because of voids resulting from compression molding ofthe powder material, and hence improvement of superconducting propertiesis restricted.

Reports by R. S. Feigelson et al., SCIENCE vol. 240, 17 June 1988, pp.1642-1645 and by G. F. de la Fuente et al., MRS SPRING MEETING, April1989 each disclose a method of preparing a superconductor such as aBi--Sr--Ca--Cu--O ceramic superconductor or a Bi--Pb--Sr--Ca--Cu--Osuperconductor in the form of fiber by a laser pedestal growth method.

However, in the conventional method of preparing a Bi--Sr--Ca--Cu--Oceramic superconductor using the laser pedestal growth method, thegrowth rate must be reduced in order to align superconducting phases.Thus, this method is has inferior productivity in terms of applicationto a wire or the like. Further, although a Bi--Sr--Ca--Cu--O ceramicsuperconductor obtained according to the conventional method attains ahigh critical current density (J_(c)) in a zero magnetic field, thecritical current density is reduced in a magnetic field. Such aphenomenon causes a significant problem in application to a magnet orthe like, which is used in a magnetic field.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a ceramicsuperconductor, which exhibits a high critical current density even in amagnetic field, and a method of preparing the same.

A Bi--Sr--Ca--Cu--O ceramic superconductor according to the presentinvention is characterized in that 0112 phases having a composition ofBi:Sr:Ca:Cu=0:1:1:2, i.e., the so-called Bi deficient phases, are finelydispersed in a 2212-phase matrix having a composition ofBi:Sr:Ca:Cu=2:2:1:2, of which c-axis is oriented perpendicular to agrowth direction.

The 2212-phase matrix preferably has heterogeneous compositiondistribution around the 0112 phases.

The method according to the present invention is adapted to prepare aBi--Sr--Ca--Cu--O ceramic superconductor by the laser pedestal growthmethod, and characterized in that crystals are grown under conditionssatisfying:

    G/R≧1 and G·R≧10000

where G (K/cm) represents the temperature gradient at a solid-liquidinterface and R (mm/h) represents the rate of crystal growth, and thegrown crystals are annealed in an atmosphere having oxygen partialpressure of at least 0.05 atm. within a temperature-range of 800° C. to860° C., for at least 2 hours.

The crystal structure of a Bi--Sr--Ca--Cu--O ceramic superconductor,i.e., the low-temperature superconducting phase of a bismuth basedsuperconductor, has a strong two-dimensionally, and its superconductingproperties such as the critical current density are strongly anisotropicdue to crystal orientation. Therefore, a high critical current densitycannot be obtained unless the crystals are oriented in order along thea-b plane, for example. However, even if a high critical current densityis attained by such orientation of the crystals, the low-temperaturesuperconducting phase of the bismuth based superconductor has inferiorforce of pinning penetrating flux lines as compared with a yttrium basedsuperconductor etc., and its critical current density is significantlyreduced in a magnetic field.

The inventive ceramic superconductor has a crystal structure such that0112 phases are finely dispersed in a 2212-phase matrix with its c-axisoriented perpendicular to a growth direction and such that its criticalcurrent density is less reduced in a magnetic field.

A Bi--Sr--Ca--Cu--O ceramic superconductor having such a crystalstructure can be obtained by growing crystals in a laser pedestal growthmethod under conditions satisfying:

    G/R≧1 and G·R≧10000

where G (K/cm) represents the temperature gradient at a solid-liquidinterface and R (mm/h) represents the rate of crystal growth, andannealing the grown crystals in an atmosphere having oxygen partialpressure of at least 0.05 atm. within a temperature range of 800° to860° C. for at least 2 hours.

FIG. 1 is a longitudinal sectional view showing an unannealed state ofcrystals which are grown according to the inventive method. Referring toFIG. 1, Bi deficient phases 1, which are 0112 phases, are oriented inthe growth direction, and other decomposition product phases 2 existaround the Bi deficient phases 1. The Bi deficient phases 1 containsmaller amounts of Bi, while the other decomposition product phases 2contain larger amounts of Bi in return.

FIG. 2 is a longitudinal sectional view showing an annealed state of thecrystals which are grown according to the inventive method. The crystalsshown in FIG. 1 are annealed under the aforementioned conditions, toattain the crystal structure shown in FIG. 2. 2212 phases 3, which arelow-temperature superconducting phases, are grown in the growthdirection, while 0112 phases 4, which are residual Bi deficient phases,are finely dispersed in the 2212 phases 3. The 2212 phases 3 are formedby reaction between the Bi deficient phases 1 and the otherdecomposition product phases 2 shown in FIG. 1. While complete 2212phases are formed at the interfaces between the Bi deficient phases 1and the other decomposition product phases 2, those formed in centralportions of the Bi deficient phases 1 away from the interfaces haveincomplete compositions. Therefore, the 2212 phases 3 contain smalleramounts of Bi in the vicinity of the 0112 phases and larger amounts ofBi around the peripheries of the 2212 phases. Thus, the 2212-phasematrix has heterogeneous composition distribution in the crystalstructure of the superconductor prepared according to the inventivemethod. Such heterogeneous composition distribution is generally called"compositional fluctuation" of 2212 phases. It seems that the inventivesuperconductor exhibits a high critical current density in a magneticfield since the finely dispersed residual 0112 phases and thecomposition fluctuation serve as pinning centers for the aforementionedflux lines. Further, since the 2212-phase matrix is oriented in thegrowth direction, the superconductor readily carries current as a wholeand exhibits a high critical current density in a zero magnetic field,while the critical current density is not much reduced but remains at ahigh level in a magnetic field.

FIG. 3 shows critical current densities of crystals, which are grownaccording to the inventive method, measured in magnetic fields. Even ifexternal magnetic fields are increased, the inventive superconductorsexhibit higher critical current densities as compared with conventionalsuperconductors which are in crystal states solidified and orienteddirectly from molten states, as shown in FIG. 3.

Unannealed crystals prepared according to the inventive method arecharacterized in that Bi deficient phases are oriented in the growthdirection. It is possible to obtain a superconductor whosesuperconducting phases are oriented in a fine crystal structure byannealing such crystals with the Bi deficient phases being oriented inthe growth direction. It has been found that orientation ofdecomposition product phases other than the Bi deficient phases is notof much importance in the unannealed crystal structure but excellentorientation of the superconducting phases can be attained so far as theBi deficient phases are oriented.

In order to orient the Bi deficient phases, i.e., 0112 phases in theunannealed state, i.e., the as-grown state, the ratio G/R of thetemperature gradient G to the growth rate R must be increased. Namely,it is possible to orient the Bi deficient phases by increasing the ratioG/R. In order to obtain the Bi deficient phases as fine structures,further, the product G-R of the temperature gradient G and the growthrate R must be increased, so that a sufficient cooling rate is ensuredto obtain the Bi deficient phases as finely oriented structures.

If the ratio G/R is less than 1, it is impossible to attain orientationof the Bi deficient phases, or crystal growth itself is disabled. If theproduct G-R is less than 10000, on the other hand, it is difficult tofinely disperse the Bi deficient phases, which are formed as coarsestructures.

The crystals grown with fine orientation of the Bi deficient phases areannealed under the aforementioned conditions, in order to obtain asuperconductor which comprises a low-temperature superconducting phasematrix, i.e., a 2212-phase matrix, and Bi deficient phases, i.e., 0112phases finely dispersed in the matrix. As to the annealing conditions,the oxygen partial pressure is set to be at least 0.05 atm. since theamount of oxygen contained in the low-temperature superconducting phasesis reduced and a sufficient critical temperature (T_(c)) cannot beobtained if the oxygen partial pressure is less than 0.05 atm. Theannealing temperature is set in a range of 800° to 860° C. sinceformation of the low-temperature superconducting phases will notsufficiently progress if the annealing temperature is less than 800° C.,while the crystals are remelted and absolutely changed in structure ifthe annealing temperature exceeds 860° C. The annealing is performed forat least 2 hours since formation of the low-temperature superconductingphases will not sufficiently progress if the annealing time is less than2 hours.

The superconductor according to the present invention exhibits a highcritical current density, which is not much reduced but remains at ahigh level even in a magnetic field. Therefore, the inventivesuperconductor can be employed as a wire which is available in amagnetic field, so that the same can be applied to a magnet which isdrivable in liquid nitrogen.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view showing an unannealed state ofcrystals which are grown according to the inventive method;

FIG. 2 is a longitudinal sectional view showing an annealed state of thecrystals which are grown according to the inventive method; and

FIG. 3 illustrates critical current densities of crystals, which aregrown according to the inventive method, measured in magnetic fields.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Raw material bars having a composition of Bi₂ Sr₂ Ca₁ CU₂ O_(x) wereemployed to grow crystal fibers of 0.4 in diameter by a laser pedestalgrowth method, under conditions of temperature gradients and growthrates shown in Table 1. The as-grown crystals were annealed in anatmosphere under the atmospheric air (oxygen partial pressure: 0.2 atm.)with annealing conditions shown in Table 1. Mean particle diameters andvolume percentages of Bi deficient phases contained in the annealedcrystals were evaluated with a transmission electron microscope.Further, critical current densities were measured in liquid nitrogen,under conditions of zero magnetic fields and magnetic fields of 500 Gwhich were applied perpendicularly to current carrying directions. Table1 also shows the results.

                                      TABLE 1                                     __________________________________________________________________________                                Mean  Volume                                                                  Particle                                                                            Percentage                                         Temperature                                                                          Growth        of Bi of Bi                                              Gradient                                                                             Rate Annealing                                                                              Deficient                                                                           Deficient                                                                           Jc     Jc                             No.    (°C./cm)                                                                      (mm/h)                                                                             Condition                                                                              Phase Phase (A/cm.sup.2, OG)                                                                     (A/cm.sup.2, 500               __________________________________________________________________________                                                   G)                             Inventive                                                                     Sample                                                                        1      1000   150  840° C.                                                                    × 90 h                                                                       0.3                                                                              μm                                                                            10%   20,000 12,000                         2      2000   700  800° C.                                                                    × 3 h                                                                        0.1                                                                              μm                                                                            15%   25,000 15,000                         3      2500   1000 820° C.                                                                    × 10 h                                                                       0.01                                                                             μm                                                                            12%   50,000 30,000                         4      3000   2000 850° C.                                                                    × 5 h                                                                        0.02                                                                             μm                                                                            18%   40,000 30,000                         5      2000   1000 830° C.                                                                    × 30 h                                                                       0.005                                                                            μm                                                                            20%   60,000 40,000                         6      1500   500  840° C.                                                                    × 80 h                                                                       0.008                                                                            μm                                                                            20%   70,000 60,000                         Comparative                                                                   Sample                                                                        7      1000   200  750° C.                                                                    × 3 h                                                                        20 μm                                                                            10%   500    0                              8      1000   200  870° C.                                                                    × 1 h                                                                        Multiphase Structure                                                                      0      --                             9      1000   5    840° C.                                                                    × 200 h                                                                      30 μm                                                                            20%   1,500  200                            10     1000   1200 Ungrowable                                                 __________________________________________________________________________

As clearly understood from Table 1, the samples Nos. 1 to 6 preparedaccording to the inventive method contained fine Bi deficient phases ofsmall mean particle diameters, and exhibited high critical currentdensities in the magnetic fields of 500 G. On the other hand, thecomparative sample No. 7, which was annealed at a lower temperature,contained Bi deficient phases of a large mean particle diameter, andexhibited a low critical current density. The comparative sample No. 8,which was annealed at a higher temperature, was in a multiphasestructure, and revealed no superconductivity. The comparative sample No.9, which had a low product G·R of 5000 below the inventive range,contained Bi deficient phases of a large mean particle diameter, andexhibited a small critical current density. In the comparative sampleNo. 10 having a ratio G/R of less than 1, it was impossible to attaincrystal growth.

EXAMPLE 2

A raw material bar having a composition of Bi₂ Sr₂ Ca₁ Cu₂ O_(x) wasemployed to grow a crystal fiber of 0.5 mm in diameter by a laserpedestal growth method under conditions of a temperature gradient of1500° C./cm and a growth rate of 300 mm/h. The as-obtained crystal fiberwas annealed at 840° C. for 50 hours, and its longitudinal section wasexposed by polishing, to linearly analyze growth-perpendicularcomponents with an X-ray microanalyzer of 50 Å in beam diameter.

In portions of low-temperature superconducting phases, compositionfluctuation was recognized in a cycle of 1 μM. The width of thisfluctuation was between Bi₂ Sr₂ Ca₀.1 Cu₁.2 O_(x) and Bi₂ Sr₂ Ca₁ Cu₂O_(x).

The portions of the low-temperature superconducting phases of thecrystal fiber were oriented in the growth direction. The crystal fiberexhibited a critical current density of 30,000 A/cm² in a zero magneticfield in liquid nitrogen. When a magnetic field of 1000 G was appliedperpendicularly to the current, the crystal fiber exhibited a criticalcurrent density of 18,000 A/cm².

The crystal fiber was observed with a transmission electron microscope,and it was confirmed that Bi deficient phases of 0.01 μm in meanparticle diameter were finely dispersed in the low-temperaturesuperconducting phase matrix with a volume percentage of 11%.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A ceramic superconductor of Bi--Sr--Ca--Cu--comprising 0112 phases dispersed in a 2212-phase matrix, wherein said2212 phase matrix has its c-axis oriented perpendicular to the growthdirection of the matrix and wherein said 2212 phase matrix has aheterogeneous composition distribution such that the 2212 phase containssmaller amounts of Bi in the vicinity of the 0112 phases and largeramounts of Bi around the periphery of the 2212 phase.
 2. A ceramicsuperconductor in accordance with claim 1, wherein said heterogeneouscomposition distribution is in the range of Bi₂ Sr₂ Ca₀.1 Cu₁.2 O_(x) toBi₂ Sr₂ Ca₁ Cu₂ O_(x).